1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, which ignites fuel in a combustion chamber by compressing an air-fuel mixture, and particularly relates to a control system which estimates a cetane number of the fuel used by the internal combustion engine.
2. Description of the Related Art
Japanese Patent Laid-open No. 2004-340026 (JP '026) discloses a method of estimating a cetane number of the fuel used in the internal combustion engine which burns the fuel by compression ignition using a relatively inexpensive sensor. Specifically, the following methods are disclosed in JP '026: 1) a method which uses an exciting force sensor for detecting an exciting force of the engine; 2) a method which uses a torque sensor for detecting an output torque of the engine; 3) a method which uses a CO sensor for detecting a concentration of CO (carbon monoxide) in exhaust gases; and 4) a method for determining the cetane number based on an angular accelerating speed calculated from a change in the engine rotational speed.
Among the above-described methods disclosed in JP '026, methods 1) to 3) require special sensors which are not normally used in engine control. While method 4) determines the cetane number based on an output of the engine rotational speed sensor, it must detect the rotational speed using a very short sampling period (i.e., a period corresponding to about 2 or 3 degrees of the crank angle).